Distinct Roles for Spontaneous and Visual Activity in Remodeling of the Retinogeniculate Synapse

A single-cell transcriptomic atlas of sensory-dependent gene expression in developing mouse visual cortex

Sensory experience drives the maturation of neural circuits during postnatal brain development through molecular mechanisms that remain to be fully elucidated. One likely mechanism involves the sensory-dependent expression of genes that encode direct mediators of circuit remodeling within developing cells. To identify potential drivers of sensory-dependent synaptic development, we generated a single-nucleus RNA sequencing dataset describing the transcriptional responses of cells in the mouse visual cortex to sensory deprivation or to stimulation during a developmental window when visual input is necessary for circuit refinement. We sequenced 118,529 nuclei across 16 neuronal and non-neuronal cell types isolated from control, sensory deprived and sensory stimulated mice, identifying 1268 sensory-induced genes within the developing brain. While experience elicited transcriptomic changes in all cell types, excitatory neurons in layer 2/3 exhibited the most robust changes, and the sensory-induced genes in these cells are poised to strengthen synapse-to-nucleus crosstalk and to promote cell type-specific axon guidance pathways. Altogether, we expect this dataset to significantly broaden our understanding of the molecular mechanisms through which sensory experience shapes neural circuit wiring in the developing brain.

Experience influences the refinement of feature selectivity in the mouse primary visual thalamus

Neurons exhibit selectivity for specific features: a property essential for extracting and encoding relevant information in the environment. This feature selectivity is thought to be modifiable by experience at the level of the cortex. Here, we demonstrate that selective exposure to a feature during development can alter the population representation of that feature in the primary visual thalamus. This thalamic plasticity is not due to changes in corticothalamic inputs and is blocked in mutant mice that exhibit deficits in retinogeniculate refinement, suggesting that plasticity is a direct result of changes in feedforward connectivity. Notably, experience-dependent changes in thalamic feature selectivity also occur in adult animals, although these changes are transient, unlike in juvenile animals, where they are long lasting. These results reveal an unexpected degree of plasticity in the visual thalamus and show that salient environmental features can be encoded in thalamic circuits during a discrete developmental window.

Visual activity enhances neuronal excitability in thalamic relay neurons

Amblyopia, a highly prevalent loss of visual acuity, is classically thought to result from cortical plasticity. The dorsal lateral geniculate nucleus (dLGN) has long been held to act as a passive relay for visual information, but recent findings suggest a largely underestimated functional plasticity in the dLGN. However, the cellular mechanisms supporting this plasticity have not yet been explored. We show here that monocular deprivation (MD), an experimental model of amblyopia, reduces the intrinsic excitability of dLGN cells. Furthermore, dLGN neurons exhibit long-term potentiation of their intrinsic excitability (LTP-IE) when suprathreshold afferent retinal inputs are stimulated at 40 hertz or when spikes are induced with current injection. LTP-IE is observed after eye opening, requires calcium influx, is expressed through the down-regulation of Kv1 channels, and is altered following MD. In conclusion, our study provides the first evidence for intrinsic plasticity in dLGN neurons induced by natural stimuli.

Purkinje cell ablation and Purkinje cell-specific deletion of Tsc1 in the developing cerebellum strengthen cerebellothalamic synapses

Cerebellar damage early in life often causes long-lasting motor, social and cognitive impairments, suggesting the roles of the cerebellum in developing a broad spectrum of behaviours. This recent finding has promoted research on how cerebellar damage affects the development of the cerebral cortex, the brain region responsible for higher-order control of all behaviours. However, the cerebral cortex is not directly connected to the cerebellum. The thalamus is a major direct target of the cerebellar nuclei, conveying cerebellar signals to the cerebral cortex. Despite its crucial position in cerebello-cerebral interaction, thalamic susceptibility to cerebellar damage remains largely unclear. Here, we studied the consequences of early cerebellar perturbation on thalamic development. Whole-cell patch-clamp recordings showed that the synaptic organization of the cerebellothlamic circuit is similar to that of the primary sensory thalamus, in which aberrant sensory activity alters synaptic circuit formation. The ablation of Purkinje cells in the developing cerebellum strengthened cerebellothalamic synapses and enhanced thalamic suprathreshold activities. Purkinje-cell specific deletion of tuberous sclerosis complex subunit 1 (Tsc1), an autism-associated gene for which the protein product negatively regulates the mammalian target of rapamycin, also strengthened cerebellothalamic synapses. However, this strengthening occurred only in homozygous deletion, whereas both homozygous and hemizygous deletion are known to cause autism-like behaviours. These results suggest that, although the cerebellothalamic projection is vulnerable to disturbances in the developing cerebellar cortex, other changes may also drive the behavioural consequences of early cerebellar perturbation. KEY POINTS: Cerebellar damage early in life often causes motor, social and cognitive impairments, suggesting the roles of the cerebellum in developing a broad spectrum of behaviours. Recent studies focus on how the developing cerebellum affects the formation and function of the cerebral cortex, the higher-order centre for all behaviours. However, the cerebellum does not directly connect to the cerebral cortex. Here, we studied the consequences of early cerebellar perturbation on the thalamus because it is a direct postsynaptic target of the cerebellum, sending cerebellar signals to the cerebral cortex. Loss of cerebellar Purkinje cells, which are commonly associated with various neurological disorders, strengthened cerebellothalamic synapses, suggesting the vulnerability of the thalamus to substantial disturbance in the developing cerebellum. Purkinje cell-specific loss of tuberous sclerosis complex-1, a negative regulator of mammalian target of rapamycin, is an established mouse model of autism. This mouse model also showed strengthened cerebellothalamic synapses.

Activity-Dependent Synapse Refinement: From Mechanisms to Molecules

The refinement of immature neuronal networks into efficient mature ones is critical to nervous system development and function. This process of synapse refinement is driven by the neuronal activity-dependent competition of converging synaptic inputs, resulting in the elimination of weak inputs and the stabilization of strong ones. Neuronal activity, whether in the form of spontaneous activity or experience-evoked activity, is known to drive synapse refinement in numerous brain regions. More recent studies are now revealing the manner and mechanisms by which neuronal activity is detected and converted into molecular signals that appropriately regulate the elimination of weaker synapses and stabilization of stronger ones. Here, we highlight how spontaneous activity and evoked activity instruct neuronal activity-dependent competition during synapse refinement. We then focus on how neuronal activity is transformed into the molecular cues that determine and execute synapse refinement. A comprehensive understanding of the mechanisms underlying synapse refinement can lead to novel therapeutic strategies in neuropsychiatric diseases characterized by aberrant synaptic function.

Developmental refinement of the active zone nanotopography and axon wiring at the somatosensory thalamus

Functional refinement of neural circuits is a crucial developmental process in the brain. However, how synaptic maturation and axon wiring proceed cooperatively to establish reliable signal transmission is unclear. Here, we combined nanotopography of release machinery at the active zone (AZ), nanobiophysics of neurotransmitter release, and single-neuron reconstruction of axon arbors of lemniscal fibers (LFs) in the developing mouse somatosensory thalamus. With development, the cluster of Cav2.1 enlarges and translocates closer to vesicle release sites inside the bouton, and LFs drastically shrink their arbors and form larger boutons on the perisomatic region of target neurons. Experimentally constrained simulations show that the nanotopography of mature synapses enables not only rapid vesicular release but also reliable transmission following repetitive firing. Sensory deprivation impairs the developmental shift of molecular nanotopography and axon wiring. Thus, we uncovered the cooperative nanotopographical and morphological mechanisms underlying the developmental establishment of reliable synaptic transmission.

Synaptic Pruning by Microglia: Lessons from Genetic Studies in Mice

BACKGROUND

Neural circuits are subjected to refinement throughout life. The dynamic addition and elimination (pruning) of synapses are necessary for maturation of neural circuits and synaptic plasticity. Due to their phagocytic nature, microglia have been considered as the primary mediators of synaptic pruning. Synaptic pruning can strengthen an active synapse by removing excess weaker synapses during development. Inappropriate synaptic pruning can often influence a disease outcome or an injury response.

SUMMARY

This review offers a focused discussion on microglial roles in synaptic pruning, based on the evidence gathered from genetic manipulations in mice. Genetically labeled microglia and synapses often allow assessment of their interactions in real time. Further manipulations involving synaptically localized molecules, neuronally or glial-derived diffusible factors, and their respective cognate receptors in microglia provide critical evidence in support of a direct role of microglia in synaptic pruning.

KEY MESSAGE

We discuss microglial contact-dependent "eat-me," "don't-eat-me," and "find-me" signals, as well as recently identified noncontact pruning, under the contexts of neural circuit, brain region, developmental window, and an injury or a disease state.

A novel developmental critical period of orexinergic signaling in the primary visual thalamus

The orexinergic system of the lateral hypothalamus plays crucial roles in arousal, feeding behavior, and reward modulation. Most research has focused on adult rodents, overlooking orexins' potential role in the nervous system development. This study, using electrophysiological and molecular tools, highlights importance of orexinergic signaling in the postnatal development of the rodent dorsolateral geniculate nucleus (DLG), a primary visual thalamic center. Orexin activation of DLG thalamocortical neurons occurs in a brief seven-day window around eye-opening, concurrent to transient OX2 receptor expression. Blocking OX2 receptors during this period reduces sensitivity of DLG neurons to green and blue light and lowers spontaneous firing rates in adulthood. This research reveals critical and temporally confined role of orexin signaling in postnatal brain development, emphasizing its contribution to experience-dependent refinement in the DLG and its long-term impact on visual function.

CSMD1 regulates brain complement activity and circuit development

Complement proteins facilitate synaptic elimination during neurodevelopmental pruning, but neural complement regulation is not well understood. CUB and Sushi Multiple Domains 1 (CSMD1) can regulate complement activity in vitro, is expressed in the brain, and is associated with increased schizophrenia risk. Beyond this, little is known about CSMD1 including whether it regulates complement activity in the brain or otherwise plays a role in neurodevelopment. We used biochemical, immunohistochemical, and proteomic techniques to examine the regional, cellular, and subcellular distribution as well as protein interactions of CSMD1 in the brain. To evaluate whether CSMD1 is involved in complement-mediated synapse elimination, we examined Csmd1-knockout mice and CSMD1-knockout human stem cell-derived neurons. We interrogated synapse and circuit development of the mouse visual thalamus, a process that involves complement pathway activity. We also quantified complement deposition on synapses in mouse visual thalamus and on cultured human neurons. Finally, we assessed uptake of synaptosomes by cultured microglia. We found that CSMD1 is present at synapses and interacts with complement proteins in the brain. Mice lacking Csmd1 displayed increased levels of complement component C3, an increased colocalization of C3 with presynaptic terminals, fewer retinogeniculate synapses, and aberrant segregation of eye-specific retinal inputs to the visual thalamus during the critical period of complement-dependent refinement of this circuit. Loss of CSMD1 in vivo enhanced synaptosome engulfment by microglia in vitro, and this effect was dependent on activity of the microglial complement receptor, CR3. Finally, human stem cell-derived neurons lacking CSMD1 were more vulnerable to complement deposition. These data suggest that CSMD1 can function as a regulator of complement-mediated synapse elimination in the brain during development.

An early enriched experience drives targeted microglial engulfment of miswired neural circuitry during a restricted postnatal period

Brain function is critically dependent on correct circuit assembly. Microglia are well-known for their important roles in immunological defense and neural plasticity, but whether they can also mediate experience-induced correction of miswired circuitry is unclear. Ten-m3 knockout (KO) mice display a pronounced and stereotyped visuotopic mismapping of ipsilateral retinal inputs in their visual thalamus, providing a useful model to probe circuit correction mechanisms. Environmental enrichment (EE) commenced around birth, but not later in life, can drive a partial correction of the most mismapped retinal inputs in Ten-m3 KO mice. Here, we assess whether enrichment unlocks the capacity for microglia to selectively engulf and remove miswired circuitry, and the timing of this effect. Expression of the microglial-associated lysosomal protein CD68 showed a clear enrichment-driven, spatially restricted change which had not commenced at postnatal day (P)18, was evident at P21, more robust at P25, and had ceased by P30. This was observed specifically at the corrective pruning site and was absent at a control site. An engulfment assay at the corrective pruning site in P25 mice showed EE-driven microglial-uptake of the mismapped axon terminals. This was temporally and spatially specific, as no enrichment-driven microglial engulfment was seen in P18 KO mice, nor the control locus. The timecourse of the EE-driven corrective pruning as determined anatomically, aligned with this pattern of microglia reactivity and engulfment. Collectively, these findings show experience can drive targeted microglial engulfment of miswired neural circuitry during a restricted postnatal window. This may have important therapeutic implications for neurodevelopmental conditions involving aberrant neural connectivity.

A single-cell transcriptomic atlas of sensory-dependent gene expression in developing mouse visual cortex

Sensory experience drives the refinement and maturation of neural circuits during postnatal brain development through molecular mechanisms that remain to be fully elucidated. One likely mechanism involves the sensory-dependent expression of genes that encode direct mediators of circuit remodeling within developing cells. However, while studies in adult systems have begun to uncover crucial roles for sensory-induced genes in modifying circuit connectivity, the gene programs induced by brain cells in response to sensory experience during development remain to be fully characterized. Here, we present a single-nucleus RNA-sequencing dataset describing the transcriptional responses of cells in mouse visual cortex to sensory deprivation or sensory stimulation during a developmental window when visual input is necessary for circuit refinement. We sequenced 118,529 individual nuclei across sixteen neuronal and non-neuronal cortical cell types isolated from control, sensory deprived, and sensory stimulated mice, identifying 1,268 unique sensory-induced genes within the developing brain. To demonstrate the utility of this resource, we compared the architecture and ontology of sensory-induced gene programs between cell types, annotated transcriptional induction and repression events based upon RNA velocity, and discovered Neurexin and Neuregulin signaling networks that underlie cell-cell interactions via CellChat . We find that excitatory neurons, especially layer 2/3 pyramidal neurons, are highly sensitive to sensory stimulation, and that the sensory-induced genes in these cells are poised to strengthen synapse-to-nucleus crosstalk by heightening protein serine/threonine kinase activity. Altogether, we expect this dataset to significantly broaden our understanding of the molecular mechanisms through which sensory experience shapes neural circuit wiring in the developing brain.

Molecular mechanisms underlying microglial sensing and phagocytosis in synaptic pruning

Microglia are the main non-neuronal cells in the central nervous system that have important roles in brain development and functional connectivity of neural circuits. In brain physiology, highly dynamic microglial processes are facilitated to sense the surrounding environment and stimuli. Once the brain switches its functional states, microglia are recruited to specific sites to exert their immune functions, including the release of cytokines and phagocytosis of cellular debris. The crosstalk of microglia between neurons, neural stem cells, endothelial cells, oligodendrocytes, and astrocytes contributes to their functions in synapse pruning, neurogenesis, vascularization, myelination, and blood-brain barrier permeability. In this review, we highlight the neuron-derived "find-me," "eat-me," and "don't eat-me" molecular signals that drive microglia in response to changes in neuronal activity for synapse refinement during brain development. This review reveals the molecular mechanism of neuron-microglia interaction in synaptic pruning and presents novel ideas for the synaptic pruning of microglia in disease, thereby providing important clues for discovery of target drugs and development of nervous system disease treatment methods targeting synaptic dysfunction.

Sleep regulation and host genetics

Due to the multifactorial and complex nature of rest, we focus on phenotypes related to sleep. Sleep regulation is a multifactorial process. In this chapter, we focus on those phenotypes inherent to sleep that are highly prevalent in the population, and that can be modulated by lifestyle, such as sleep quality and duration, insomnia, restless leg syndrome and daytime sleepiness. We, therefore, leave in the background those phenotypes that constitute infrequent pathologies or for which the current level of scientific evidence does not favour the implementation of practical approaches of this type. Similarly, the regulation of sleep quality is intimately linked to the regulation of the circadian rhythm. Although this relationship is discussed in the sections that require it, the in-depth study of circadian rhythm regulation at the molecular level deserves a separate chapter, and this is how it is dealt with in this volume.

Mapping the Retina onto the Brain

Vision begins in the retina, which extracts salient features from the environment and encodes them in the spike trains of retinal ganglion cells (RGCs), the output neurons of the eye. RGC axons innervate diverse brain areas (>50 in mice) to support perception, guide behavior, and mediate influences of light on physiology and internal states. In recent years, complete lists of RGC types (∼45 in mice) have been compiled, detailed maps of their dendritic connections drawn, and their light responses surveyed at scale. We know less about the RGCs' axonal projection patterns, which map retinal information onto the brain. However, some organizing principles have emerged. Here, we review the strategies and mechanisms that govern developing RGC axons and organize their innervation of retinorecipient brain areas.

A developmental critical period for ocular dominance plasticity of binocular neurons in mouse superior colliculus

Detecting visual features in the environment is crucial for animals' survival. The superior colliculus (SC) is implicated in motion detection and processing, whereas how the SC integrates visual inputs from the two eyes remains unclear. Using in vivo electrophysiology, we show that mouse SC contains many binocular neurons that display robust ocular dominance (OD) plasticity in a critical period during early development, which is similar to, but not dependent on, the primary visual cortex. NR2A- and NR2B-containing N-methyl-D-aspartate (NMDA) receptors play an essential role in the regulation of SC plasticity. Blocking NMDA receptors can largely prevent the impairment of predatory hunting caused by monocular deprivation, indicating that maintaining the binocularity of SC neurons is required for efficient hunting behavior. Together, our studies reveal the existence and function of OD plasticity in SC, which broadens our understanding of the development of subcortical visual circuitry relating to motion detection and predatory hunting.

Spatiotemporal Mapping and Molecular Basis of Whole-brain Circuit Maturation

Brain development is highly dynamic and asynchronous, marked by the sequential maturation of functional circuits across the brain. The timing and mechanisms driving circuit maturation remain elusive due to an inability to identify and map maturing neuronal populations. Here we create DevATLAS (Developmental Activation Timing-based Longitudinal Acquisition System) to overcome this obstacle. We develop whole-brain mapping methods to construct the first longitudinal, spatiotemporal map of circuit maturation in early postnatal mouse brains. Moreover, we uncover dramatic impairments within the deep cortical layers in a neurodevelopmental disorders (NDDs) model, demonstrating the utility of this resource to pinpoint when and where circuit maturation is disrupted. Using DevATLAS, we reveal that early experiences accelerate the development of hippocampus-dependent learning by increasing the synaptically mature granule cell population in the dentate gyrus. Finally, DevATLAS enables the discovery of molecular mechanisms driving activity-dependent circuit maturation.

Synapse Regulation

Microglia are the resident immune cells of the brain. As such, they rapidly detect changes in normal brain homeostasis and accurately respond by fine-tuning in a tightly regulated manner their morphology, gene expression, and functional behavior. Depending on the nature of these changes, microglia can thicken and retract their processes, proliferate and migrate, release numerous signaling factors and compounds influencing neuronal physiology (e.g., cytokines and trophic factors), in addition to secreting proteases able to transform the extracellular matrix, and phagocytosing various types of cellular debris, etc. Because microglia also transform rapidly (on a time scale of minutes) during experimental procedures, studying these very special cells requires methods that are specifically non-invasive. The development of such methods has provided unprecedented insights into the roles of microglia during normal physiological conditions. In particular, transcranial two-photon in vivo imaging revealed that presumably "resting" microglia continuously survey the brain parenchyma with their highly motile processes, in addition to modulating their structural and functional interactions with neuronal circuits along the changes in neuronal activity and behavioral experience occurring throughout the lifespan. In this chapter, we will describe how surveillant microglia interact with synaptic elements and modulate the number, maturation, function, and plasticity of synapses in the healthy developing, mature, and aging brain, with consequences on neuronal activity, learning and memory, and the behavioral outcome.

Maternal diet during early gestation influences postnatal taste activity-dependent pruning by microglia

A key process in central sensory circuit development involves activity-dependent pruning of exuberant terminals. Here, we studied gustatory terminal field maturation in the postnatal mouse nucleus of the solitary tract (NST) during normal development and in mice where their mothers were fed a low NaCl diet for a limited period soon after conception. Pruning of terminal fields of gustatory nerves in controls involved the complement system and is likely driven by NaCl-elicited taste activity. In contrast, offspring of mothers with an early dietary manipulation failed to prune gustatory terminal fields even though peripheral taste activity developed normally. The ability to prune in these mice was rescued by activating myeloid cells postnatally, and conversely, pruning was arrested in controls with the loss of myeloid cell function. The altered pruning and myeloid cell function appear to be programmed before the peripheral gustatory system is assembled and corresponds to the embryonic period when microglia progenitors derived from the yolk sac migrate to and colonize the brain.

Loss of Purkinje cells in the developing cerebellum strengthens the cerebellothalamic synapses

Cerebellar damage early in life often causes long-lasting motor, social, and cognitive impairments, suggesting the roles of the cerebellum in developing a broad spectrum of behaviors. This recent finding has promoted research on how cerebellar damage affects the development of the cerebral cortex, the brain region responsible for higher-order control of all behaviors. However, the cerebral cortex is not directly connected to the cerebellum. The thalamus is the direct postsynaptic target of the cerebellum, sending cerebellar outputs to the cerebral cortex. Despite its crucial position in cerebello-cerebral interaction, thalamic susceptibility to cerebellar damage remains largely unclear. Here, we studied the consequences of early cerebellar perturbation on thalamic development. Whole-cell patch-clamp recordings showed that the synaptic organization of the cerebellothlamic circuit is similar to that of the primary sensory thalamus, in which aberrant sensory activity alters synaptic circuit formation. The hemizygous deletion of the tuberous sclerosis complex-1 ( Tsc1 ) gene in the Purkinje cell-known to cause Purkinje cell hypoactivity and autistic behaviors-did not alter cerebellothalamic synapses or intrinsic membrane properties of thalamic neurons. However, the ablation of Purkinje cells in the developing cerebellum strengthened the cerebellothalamic synapses and enhanced thalamic suprathreshold activities. These results suggest that the cerebellothalamic circuit is resistant to moderate perturbation in the developing cerebellum, such as the reduced firing rate of Purkinje cells, and that autistic behaviors are not necessarily linked to thalamic abnormality. Still, Purkinje cell loss alters the thalamic circuit, suggesting the vulnerability of the thalamus to substantial disturbance in the developing cerebellum.

Activity-induced MeCP2 phosphorylation regulates retinogeniculate synapse refinement

Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelopmental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MECP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here, we show that MeCP2 is phosphorylated at four residues in the mouse brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from the synapse refinement defect previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period.

Activity-dependent development of the body's touch receptors

We report a role for activity in the development of the primary sensory neurons that detect touch. Genetic deletion of Piezo2, the principal mechanosensitive ion channel in somatosensory neurons, caused profound changes in the formation of mechanosensory end organ structures and altered somatosensory neuron central targeting. Single cell RNA sequencing of Piezo2 conditional mutants revealed changes in gene expression in the sensory neurons activated by light mechanical forces, whereas other neuronal classes were less affected. To further test the role of activity in mechanosensory end organ development, we genetically deleted the voltage-gated sodium channel Nav1.6 (Scn8a) in somatosensory neurons throughout development and found that Scn8a mutants also have disrupted somatosensory neuron morphologies and altered electrophysiological responses to mechanical stimuli. Together, these findings indicate that mechanically evoked neuronal activity acts early in life to shape the maturation of the mechanosensory end organs that underlie our sense of gentle touch.

β-Catenin and SOX2 Interaction Regulate Visual Experience-Dependent Cell Homeostasis in the Developing Xenopus Thalamus

In the vertebrate brain, sensory experience plays a crucial role in shaping thalamocortical connections for visual processing. However, it is still not clear how visual experience influences tissue homeostasis and neurogenesis in the developing thalamus. Here, we reported that the majority of SOX2-positive cells in the thalamus are differentiated neurons that receive visual inputs as early as stage 47 Xenopus. Visual deprivation (VD) for 2 days shifts the neurogenic balance toward proliferation at the expense of differentiation, which is accompanied by a reduction in nuclear-accumulated β-catenin in SOX2-positive neurons. The knockdown of β-catenin decreases the expression of SOX2 and increases the number of progenitor cells. Coimmunoprecipitation studies reveal the evolutionary conservation of strong interactions between β-catenin and SOX2. These findings indicate that β-catenin interacts with SOX2 to maintain homeostatic neurogenesis during thalamus development.

Activity-Induced MeCP2 Phosphorylation Regulates Retinogeniculate Synapse Refinement

Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelopmental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MeCP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here we show that MeCP2 is phosphorylated at four residues in the brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from that previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period.

SIGNIFICANCE STATEMENT

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that predominantly affects girls. RTT is caused by loss of function mutations in a single gene MeCP2. Girls with RTT develop normally during their first year of life, but then experience neurological abnormalities including breathing and movement difficulties, loss of speech, and seizures. This study investigates the function of the MeCP2 protein in the brain, and how MeCP2 activity is modulated by sensory experience in early life. Evidence is presented that sensory experience affects MeCP2 function, and that this is required for synaptic pruning in the brain. These findings provide insight into MeCP2 function, and clues as to what goes awry in the brain when the function of MeCP2 is disrupted.

Retinal waves align the concentric orientation map in mouse superior colliculus to the center of vision

Neurons in the mouse superior colliculus (SC) are arranged in a concentric orientation map, which is aligned to the center of vision and the optic flow experienced by the mouse. The origin of this map remains unclear. Here, we propose that spontaneous retinal waves during development provide a scaffold to establish the concentric orientation map within the SC and its alignment to the optic flow. We test this hypothesis by modeling the orientation-tuned SC neurons that receive ON/OFF retinal inputs. Our model suggests that the propagation direction bias of stage III retinal waves, together with OFF-delayed responses, shapes the spatial organization of the orientation map. The OFF delay establishes orientation-tuned neurons by segregating their ON/OFF receptive subfields, the wave-like activities form the concentric pattern, and the direction biases align the map to the center of vision. Together, retinal waves may play an instructive role in establishing functional properties of single SC neurons and their spatial organization within maps.

Brainstem serotonin neurons selectively gate retinal information flow to thalamus

Retinal ganglion cell (RGC) types relay parallel streams of visual feature information. We hypothesized that neuromodulators might efficiently control which visual information streams reach the cortex by selectively gating transmission from specific RGC axons in the thalamus. Using fiber photometry recordings, we found that optogenetic stimulation of serotonergic axons in primary visual thalamus of awake mice suppressed ongoing and visually evoked calcium activity and glutamate release from RGC boutons. Two-photon calcium imaging revealed that serotonin axon stimulation suppressed RGC boutons that responded strongly to global changes in luminance more than those responding only to local visual stimuli, while the converse was true for suppression induced by increases in arousal. Converging evidence suggests that differential expression of the 5-HT1B receptor on RGC presynaptic terminals, but not differential density of nearby serotonin axons, may contribute to the selective serotonergic gating of specific visual information streams before they can activate thalamocortical neurons.

Structural and functional integration of human forebrain organoids with the injured adult rat visual system

Brain organoids created from human pluripotent stem cells represent a promising approach for brain repair. They acquire many structural features of the brain and raise the possibility of patient-matched repair. Whether these entities can integrate with host brain networks in the context of the injured adult mammalian brain is not well established. Here, we provide structural and functional evidence that human brain organoids successfully integrate with the adult rat visual system after transplantation into large injury cavities in the visual cortex. Virus-based trans-synaptic tracing reveals a polysynaptic pathway between organoid neurons and the host retina and reciprocal connectivity between the graft and other regions of the visual system. Visual stimulation of host animals elicits responses in organoid neurons, including orientation selectivity. These results demonstrate the ability of human brain organoids to adopt sophisticated function after insertion into large injury cavities, suggesting a translational strategy to restore function after cortical damage.

A Guide for the Multiplexed: The Development of Visual Feature Maps in the Brain

Neural maps are found ubiquitously in the brain, where they encode a wide range of behaviourally relevant features into neural space. Developmental studies have shown that animals devote a great deal of resources to establish consistently patterned organization in neural circuits throughout the nervous system, but what purposes maps serve beneath their often intricate appearance and composition is a topic of active debate and exploration. In this article, we review the general mechanisms of map formation, with a focus on the visual system, and then survey notable organizational properties of neural maps: the multiplexing of feature representations through a nested architecture, the interspersing of fine-scale heterogeneity within a globally smooth organization, and the complex integration at the microcircuit level that enables a high dimensionality of information encoding. Finally, we discuss the roles of maps in cortical functions, including input segregation, feature extraction and routing of circuit outputs for higher order processing, as well as the evolutionary basis for the properties we observe in neural maps.

Vision-Dependent and -Independent Molecular Maturation of Mouse Retinal Ganglion Cells

The development and connectivity of retinal ganglion cells (RGCs), the retina's sole output neurons, are patterned by activity-independent transcriptional programs and activity-dependent remodeling. To inventory the molecular correlates of these influences, we applied high-throughput single-cell RNA sequencing (scRNA-seq) to mouse RGCs at six embryonic and postnatal ages. We identified temporally regulated modules of genes that correlate with, and likely regulate, multiple phases of RGC development, ranging from differentiation and axon guidance to synaptic recognition and refinement. Some of these genes are expressed broadly while others, including key transcription factors and recognition molecules, are selectively expressed by one or a few of the 45 transcriptomically distinct types defined previously in adult mice. Next, we used these results as a foundation to analyze the transcriptomes of RGCs in mice lacking visual experience due to dark rearing from birth or to mutations that ablate either bipolar or photoreceptor cells. 98.5% of visually deprived (VD) RGCs could be unequivocally assigned to a single RGC type based on their transcriptional profiles, demonstrating that visual activity is dispensable for acquisition and maintenance of RGC type identity. However, visual deprivation significantly reduced the transcriptomic distinctions among RGC types, implying that activity is required for complete RGC maturation or maintenance. Consistent with this notion, transcriptomic alternations in VD RGCs significantly overlapped with gene modules found in developing RGCs. Our results provide a resource for mechanistic analyses of RGC differentiation and maturation, and for investigating the role of activity in these processes.

An Early Enriched Experience Drives an Activated Microglial Profile at Site of Corrective Neuroplasticity in Ten-m3 Knock-Out Mice

Environmental enrichment (EE) is beneficial for brain development and function, but our understanding of its capacity to drive circuit repair, the underlying mechanisms, and how this might vary with age remains limited. Ten-m3 knock-out (KO) mice exhibit a dramatic and stereotyped mistargeting of ipsilateral retinal inputs to the thalamus, resulting in visual deficits. We have recently shown a previously unexpected capacity for EE during early postnatal life (from birth for six weeks) to drive the partial elimination of miswired axonal projections, along with a recovery of visually mediated behavior, but the timeline of this repair was unclear. Here, we reveal that with just 3.5 weeks of EE from birth, Ten-m3 KOs exhibit a partial behavioral rescue, accompanied by pruning of the most profoundly miswired retinogeniculate terminals. Analysis suggests that the pruning is underway at this time point, providing an ideal opportunity to probe potential mechanisms. With the shorter EE-period, we found a localized increase in microglial density and activation profile within the identified geniculate region where corrective pruning was observed. No comparable response to EE was found in age-matched wild-type (WT) mice. These findings identify microglia as a potential mechanistic link through which EE drives the elimination of miswired neural circuits during early postnatal development. Activity driven, atypical recruitment of microglia to prune aberrant connectivity and restore function may have important therapeutic implications for neurodevelopmental disorders such as autistic spectrum disorder.

Loss of Retinogeniculate Synaptic Function in the DBA/2J Mouse Model of Glaucoma

Retinal ganglion cell (RGC) axons comprise the optic nerve and carry information to the dorsolateral geniculate nucleus (dLGN), which is then relayed to the cortex for conscious vision. Glaucoma is a blinding neurodegenerative disease that commonly results from intraocular pressure (IOP)-associated injury leading to RGC axonal pathology, disruption of RGC outputs to the brain, and eventual apoptotic loss of RGC somata. The consequences of elevated IOP and glaucomatous pathology on RGC signaling to the dLGN are largely unknown yet are likely to contribute to vision loss. Here, we used anatomic and physiological approaches to study the structure and function of retinogeniculate (RG) synapses in male and female DBA/2J (D2) mice with inherited glaucoma before and after IOP elevation. D2 mice showed progressive loss of anterograde optic tract transport to the dLGN and vGlut2 labeling of RGC axon terminals while patch-clamp measurements of RG synaptic function showed that synaptic transmission was reduced in 9-month and 12-month D2 mice because of the loss of individual RGC axon inputs. TC neuron dendrites had reduced Sholl complexity at 12 months, suggestive of delayed reorganization following reduced synaptic input. There was no detectable change in RGC density in 11- to 12-month D2 retinas, quantified as the number of ganglion cell layer-residing somata immuno-positive for NeuN and immuno-negative for the amacrine marker choline acetyltransferase (ChAT). Thus, observed synaptic defects appear to precede RGC somatic loss. These findings identify glaucoma-associated and IOP-associated deficits in an important subcortical RGC projection target, shedding light on processes linking IOP to vision loss.

The encoding of touch by somatotopically aligned dorsal column subdivisions

The somatosensory system decodes a range of tactile stimuli to generate a coherent sense of touch. Discriminative touch of the body depends on signals conveyed from peripheral mechanoreceptors to the brain through the spinal cord dorsal column and its brainstem target, the dorsal column nuclei (DCN)1,2. Models of somatosensation emphasize that fast-conducting low-threshold mechanoreceptors (LTMRs) innervating the skin drive the DCN3,4. However, postsynaptic dorsal column (PSDC) neurons within the spinal cord dorsal horn also collect mechanoreceptor signals and form a second major input to the DCN5-7. The significance of PSDC neurons and their contributions to the coding of touch have remained unclear since their discovery. Here we show that direct LTMR input to the DCN conveys vibrotactile stimuli with high temporal precision. Conversely, PSDC neurons primarily encode touch onset and the intensity of sustained contact into the high-force range. LTMR and PSDC signals topographically realign in the DCN to preserve precise spatial detail. Different DCN neuron subtypes have specialized responses that are generated by distinct combinations of LTMR and PSDC inputs. Thus, LTMR and PSDC subdivisions of the dorsal column encode different tactile features and differentially converge in the DCN to generate specific ascending sensory processing streams.

Developmental stage-specific spontaneous activity contributes to callosal axon projections

The developing neocortex exhibits spontaneous network activity with various synchrony levels, which has been implicated in the formation of cortical circuits. We previously reported that the development of callosal axon projections, one of the major long-range axonal projections in the brain, is activity dependent. However, what sort of activity and when activity is indispensable are not known. Here, using a genetic method to manipulate network activity in a stage-specific manner, we demonstrated that network activity contributes to callosal axon projections in the mouse visual cortex during a ‘critical period’: restoring neuronal activity during that period resumed the projections, whereas restoration after the period failed. Furthermore, in vivo Ca²⁺ imaging revealed that the projections could be established even without fully restoring highly synchronous activity. Overall, our findings suggest that spontaneous network activity is selectively required during a critical developmental time window for the formation of long-range axonal projections in the cortex.

Functional convergence of on-off direction-selective ganglion cells in the visual thalamus

In the mouse visual system, multiple types of retinal ganglion cells (RGCs) each encode distinct features of the visual space. A clear understanding of how this information is parsed in their downstream target, the dorsal lateral geniculate nucleus (dLGN), remains elusive. Here, we characterized retinogeniculate connectivity in Cart-IRES2-Cre-D and BD-CreER2 mice, which labels subsets of on-off direction-selective ganglion cells (ooDSGCs) tuned to the vertical directions and to only ventral motion, respectively. Our immunohistochemical, electrophysiological, and optogenetic experiments reveal that only a small fraction (<15%) of thalamocortical (TC) neurons in the dLGN receives primary retinal drive from these subtypes of ooDSGCs. The majority of the functionally identifiable ooDSGC inputs in the dLGN are weak and converge together with inputs from other RGC types. Yet our modeling indicates that this mixing is not random: BD-CreER+ ooDSGC inputs converge less frequently with ooDSGCs tuned to the opposite direction than with non-CART-Cre+ RGC types. Taken together, these results indicate that convergence of distinct information lines in dLGN follows specific rules of organization.

CKAMP44 controls synaptic function and strength of relay neurons during early development of the dLGN

KEY POINTS

Expression of CKAMP44 starts early during development of the dLGN and remains stable in relay neurons and interneurons. Genetic deletion of CKAMP44 decreases synaptic strength and increases silent synapse number in dLGN relay neurons. Genetic deletion of CKAMP44 increases the rate of recovery from desensitisation of AMPA receptors in dLGN relay neurons. Genetic deletion of CKAMP44 reduces synaptic short-term depression in retinogeniculate synapses. The probability to induce plateau potentials is elevated in relay neurons of CKAMP44^-/- mice. Eye-specific input segregation is unaffected in the dLGN of CKAMP44^-/- mice. Deletion of CKAMP44 mildly affects dendritic arborisation of relay neurons in the dLGN.

ABSTRACT

Relay neurons of the dorsal lateral geniculate nucleus (dLGN) receive inputs from retinal ganglion cells via retinogeniculate synapses. These connections undergo pruning in the first two weeks after eye opening. The remaining connections are strengthened several-fold by the insertion of AMPA receptors (AMPARs) into weak or silent synapses. In this study, we found that the AMPAR auxiliary subunit CKAMP44 is required for receptor insertion and function of retinogeniculate synapses during development. Genetic deletion of CKAMP44 resulted in decreased synaptic strength and a higher number of silent synapses in young (P9-11) mice. Recovery from desensitisation of AMPA receptors was faster in CKAMP44 knockout (CKAMP44^-/- ) than in wildtype mice. Moreover, loss of CKAMP44 increased the probability to induce plateau potentials, which are known to be important for eye-specific input segregation and retinogeniculate synapse maturation. The anatomy of relay neurons in the dLGN was changed in young CKAMP44^-/- mice showing a transient increase in dendritic branching that normalised during later development (P26-33). Interestingly, input segregation in young CKAMP44^-/- mice was not affected when compared to wildtype mice. These results demonstrate that CKAMP44 promotes maturation and modulates function of retinogeniculate synapses during early development of the visual system without affecting input segregation. Abstract figure legend AMPA receptor auxiliary subunit CKAMP44 influences synaptic function in retinogeniculate synapses of young mice. CKAMP44 unsilences synapses by recruiting AMPA receptors to the synapse. Furthermore, genetic deletion of CKAMP44 reduces short-term depression and increases the probability to elicit L-type Ca²⁺ channel-mediated plateau potentials. This article is protected by copyright. All rights reserved.

Epigenetic and Transcriptional Regulation of Spontaneous and Sensory Activity Dependent Programs During Neuronal Circuit Development

Spontaneous activity generated before the onset of sensory transduction has a key role in wiring developing sensory circuits. From axonal targeting, to synapse formation and elimination, to the balanced integration of neurons into developing circuits, this type of activity is implicated in a variety of cellular processes. However, little is known about its molecular mechanisms of action, especially at the level of genome regulation. Conversely, sensory experience-dependent activity implements well-characterized transcriptional and epigenetic chromatin programs that underlie heterogeneous but specific genomic responses that shape both postnatal circuit development and neuroplasticity in the adult. In this review, we focus on our knowledge of the developmental processes regulated by spontaneous activity and the underlying transcriptional mechanisms. We also review novel findings on how chromatin regulates the specificity and developmental induction of the experience-dependent program, and speculate their relevance for our understanding of how spontaneous activity may act at the genomic level to instruct circuit assembly and prepare developing neurons for sensory-dependent connectivity refinement and processing.

Development of astrocyte morphology and function in mouse visual thalamus

The rodent visual thalamus has served as a powerful model to elucidate the cellular and molecular mechanisms that underlie sensory circuit formation and function. Despite significant advances in our understanding of the role of axon-target interactions and neural activity in orchestrating circuit formation in visual thalamus, the role of non-neuronal cells, such as astrocytes, is less clear. In fact, we know little about the transcriptional identity and development of astrocytes in mouse visual thalamus. To address this gap in knowledge, we studied the expression of canonical astrocyte molecules in visual thalamus using immunostaining, in situ hybridization, and reporter lines. While our data suggests some level of heterogeneity of astrocytes in different nuclei of the visual thalamus, the majority of thalamic astrocytes appeared to be labeled in Aldh1l1-EGFP mice. This led us to use this transgenic line to characterize the neonatal and postnatal development of these cells in visual thalamus. Our data show that not only have the entire cohort of astrocytes migrated into visual thalamus by eye-opening but they also have acquired their adult-like morphology, even while retinogeniculate synapses are still maturing. Furthermore, ultrastructural, immunohistochemical, and functional approaches revealed that by eye-opening, thalamic astrocytes ensheathe retinogeniculate synapses and are capable of efficient uptake of glutamate. Taken together, our results reveal that the morphological, anatomical, and functional development of astrocytes in visual thalamus occurs prior to eye-opening and the emergence of experience-dependent visual activity.

Development and experience-dependent modulation of the defensive behaviors of mice to visual threats

Rodents demonstrate defensive behaviors such as fleeing or freezing upon recognizing a looming shadow above them. Although individuals' experiences in their habitat can modulate the defensive behavior phenotype, the effects of systematically manipulating the individual's visual experience on vision-guided defensive behaviors have not been studied. We aimed to describe the developmental process of defensive behaviors in response to visual threats and the effects of visual deprivation. We found that the probability of escape response occurrence increased 3 weeks postnatally, and then stabilized. When visual experience was perturbed by dark rearing from postnatal day (P) 21 for a week, the developmental increase in escape probability was clearly suppressed, while the freezing probability increased. Intriguingly, exposure to the looming stimuli at P28 reversed the suppression of escape response development at P35. These results clearly indicate that the development of defensive behaviors in response to looming stimuli is affected by an individual's sensory experience.

Structural and functional plasticity in the dorsolateral geniculate nucleus of mice following bilateral enucleation

Within the nervous system, plasticity mechanisms attempt to stabilize network activity following disruption by injury, disease, or degeneration. Optic nerve injury and age-related diseases can induce homeostatic-like responses in adulthood. We tested this possibility in the thalamocortical (TC) neurons in the dorsolateral geniculate nucleus (dLGN) using patch-clamp electrophysiology, optogenetics, immunostaining, and single-cell dendritic analysis following loss of visual input via bilateral enucleation. We observed progressive loss of vGlut2-positive retinal terminals in the dLGN indicating degeneration post-enucleation that was coincident with changes in microglial morphology indicative of microglial activation. Consistent with the decline of vGlut2 puncta, we also observed loss of retinogeniculate (RG) synaptic function assessed using optogenetic activation of RG axons while performing whole-cell voltage clamp recordings from TC neurons in brain slices. Surprisingly, we did not detect any significant changes in the frequency of miniature post-synaptic currents (mEPSCs) or corticothalamic feedback synapses. Analysis of TC neuron dendritic structure from single-cell dye fills revealed a gradual loss of dendrites proximal to the soma, where TC neurons receive the bulk of RG inputs. Finally, analysis of action potential firing demonstrated that TC neurons have increased excitability following enucleation, firing more action potentials in response to depolarizing current injections. Our findings show that degeneration of the retinal axons/optic nerve and loss of RG synaptic inputs induces structural and functional changes in TC neurons, consistent with neuronal attempts at compensatory plasticity in the dLGN.

Development of Functional Properties in the Early Visual System: New Appreciations of the Roles of Lateral Geniculate Nucleus

In the years following Hubel and Wiesel's first reports on ocular dominance plasticity and amblyopia, much attention has been focused on understanding the role of cortical circuits in developmental and experience-dependent plasticity. Initial studies found few differences between retinal ganglion cells and neurons in the lateral geniculate nucleus and uncovered little evidence for an impact of altered visual experience on the functional properties of lateral geniculate nucleus neurons. In the last two decades, however, studies have revealed that the connectivity between the retina and lateral geniculate nucleus is much richer than was previously appreciated, even revealing visual plasticity - including ocular dominance plasticity - in lateral geniculate nucleus neurons. Here we review the development of the early visual system and the impact of experience with a distinct focus on recent discoveries about lateral geniculate nucleus, its connectivity, and evidence for its plasticity and rigidity during development.

A short period of early life oxytocin treatment rescues social behavior dysfunction via suppression of hippocampal hyperactivity in male mice

Early sensory experiences interact with genes to shape precise neural circuits during development. This process is vital for proper brain function in adulthood. Neurological dysfunctions caused by environmental alterations and/or genetic mutation may share the same molecular or cellular mechanisms. Here, we show that early life bilateral whisker trimming (BWT) subsequently affects social discrimination in adult male mice. Enhanced activation of the hippocampal dorsal CA3 (dCA3) in BWT mice was observed during social preference tests. Optogenetic activation of dCA3 in naive mice impaired social discrimination, whereas chemogenetic silencing of dCA3 rescued social discrimination deficit in BWT mice. Hippocampal oxytocin (OXT) is reduced after whisker trimming. Neonatal intraventricular compensation of OXT relieved dCA3 over-activation and prevented social dysfunction. Neonatal knockdown of OXT receptor in dCA3 mimics the effects of BWT, and cannot be rescued by OXT treatment. Social behavior deficits in a fragile X syndrome mouse model (Fmr1 KO mice) could also be recovered by early life OXT treatment, through negating dCA3 over-activation. Here, a possible avenue to prevent social dysfunction is uncovered.

Neurotransmitter regulation rather than cell-intrinsic properties shapes the high-pass filtering properties of olfactory bulb glomeruli

GABAergic periglomerular (PG) cells in the olfactory bulb are proposed to mediate an intraglomerular 'high-pass' filter through inhibition targeted onto a glomerulus. With this mechanism, weak stimuli (e.g. an odour with a low affinity for an odourant receptor) mainly produce PG cell-driven inhibition but strong stimuli generate enough excitation to overcome inhibition. PG cells may be particularly susceptible to being activated by weak stimuli due to their intrinsically small size and high input resistance. Here, we used dual-cell patch-clamp recordings and imaging methods in bulb slices obtained from wild-type and transgenic rats with labelled GABAergic cells to test a number of predictions of the high-pass filtering model. We first directly compared the responsiveness of PG cells with that of external tufted cells (eTCs), which are a class of excitatory cells that reside in a parallel but opposing position in the glomerular circuitry. PG cells were in fact found to be no more responsive than eTCs at low levels of sensory neuron activity. While PG cells required smaller currents to be excited, this advantage was offset by the fact that a given level of sensory neuron activity produced much smaller synaptic currents. We did, however, identify other factors that shaped the excitation/inhibition balance in a manner that would support a high-pass filter, including glial glutamate transporters and presynaptic metabotropic glutamate receptors. We conclude that a single glomerulus may act as a high-pass filter to enhance the contrast between different olfactory stimuli through mechanisms that are largely independent cell-intrinsic properties. KEY POINTS: GABAergic periglomerular (PG) cells in the olfactory bulb are proposed to mediate a 'high-pass' filter at a single glomerulus that selectively blocks weak stimulus signals. Their efficacy may reflect their intrinsically small size and high input resistance, which allows them to be easily excited. It was found that PG cells were in fact no more likely to be activated by weak stimuli than excitatory neurons. PG cells fired action potentials more readily in response to a fixed current input, but this advantage for excitability was offset by small synaptic currents. Glomeruli nevertheless display an excitation/inhibition balance that can support a high-pass filter, shifting from unfavourable to favourable with increasing stimulus strength. Factors shaping the filter include glial glutamate transporters and presynaptic metabotropic glutamate receptors. It is concluded that a single glomerulus may act as a high-pass filter to enhance stimulus contrast through mechanisms that are largely independent of cell-intrinsic properties.

Adenosine A2A receptor modulates microglia-mediated synaptic pruning of the retinogeniculate pathway during postnatal development

Synapse pruning is essential not only for the developmental establishment of synaptic connections in the brain but also for the pathogenesis of neurodevelopmental and neurodegenerative disorders. However, there are no effective pharmacological means to regulate synaptic pruning during early development. Using the eye-specific segregation of the dorsal lateral geniculate nucleus (dLGN) as a model of synaptic pruning coupled with adenosine A2A receptor (A2AR) antagonism and knockout, we demonstrated while genetic deletion of the A2AR throughout the development attenuated eye-specific segregation with the attenuated microglial phagocytosis at postnatal day 5 (P5), selective treatment with the A2AR antagonist KW6002 at P2-P4 facilitated synaptic pruning of visual pathway with microglial activation, increased lysosomal activity in microglia and increased microglial engulfment of retinal ganglion cell (RGC) inputs in the dLGN at P5 (but not P10). Furthermore, KW6002-mediated facilitation of synaptic pruning was activity-dependent since tetrodotoxin (TTX) treatment abolished the KW6002 facilitation. Moreover, the A2AR antagonist also modulated postsynaptic proteins and synaptic density at early postnatal stages as revealed by the reduced immunoreactivity of postsynaptic proteins (Homer1 and metabotropic glutamate receptor 5) and colocalization of presynaptic VGlut2 and postsynaptic Homer1 puncta in the dLGN. These findings suggest that A2AR can control pruning by multiple actions involving the retinal wave, microglia engulfment, and postsynaptic stability. Thus, A2AR antagonists may represent a novel pharmacological strategy to modulate microglia-mediated synaptic pruning and treatment of neurodevelopmental disorders associated with dysfunctional pruning.

Phospholipid-flippase chaperone CDC50A is required for synapse maintenance by regulating phosphatidylserine exposure

Synaptic refinement is a critical physiological process that removes excess synapses to establish and maintain functional neuronal circuits. Recent studies have shown that focal exposure of phosphatidylserine (PS) on synapses acts as an "eat me" signal to mediate synaptic pruning. However, the molecular mechanism underlying PS externalization at synapses remains elusive. Here, we find that murine CDC50A, a chaperone of phospholipid flippases, localizes to synapses, and that its expression depends on neuronal activity. Cdc50a knockdown leads to phosphatidylserine exposure at synapses and subsequent erroneous synapse removal by microglia partly via the GPR56 pathway. Taken together, our data support that CDC50A safeguards synapse maintenance by regulating focal phosphatidylserine exposure at synapses.

Mechanoreceptor synapses in the brainstem shape the central representation of touch

Mammals use glabrous (hairless) skin of their hands and feet to navigate and manipulate their environment. Cortical maps of the body surface across species contain disproportionately large numbers of neurons dedicated to glabrous skin sensation, in part reflecting a higher density of mechanoreceptors that innervate these skin regions. Here, we find that disproportionate representation of glabrous skin emerges over postnatal development at the first synapse between peripheral mechanoreceptors and their central targets in the brainstem. Mechanoreceptor synapses undergo developmental refinement that depends on proximity of their terminals to glabrous skin, such that those innervating glabrous skin make synaptic connections that expand their central representation. In mice incapable of sensing gentle touch, mechanoreceptors innervating glabrous skin still make more powerful synapses in the brainstem. We propose that the skin region a mechanoreceptor innervates controls the developmental refinement of its central synapses to shape the representation of touch in the brain.

Activity-dependent modulation of synapse-regulating genes in astrocytes

Astrocytes regulate the formation and function of neuronal synapses via multiple signals; however, what controls regional and temporal expression of these signals during development is unknown. We determined the expression profile of astrocyte synapse-regulating genes in the developing mouse visual cortex, identifying astrocyte signals that show differential temporal and layer-enriched expression. These patterns are not intrinsic to astrocytes, but regulated by visually evoked neuronal activity, as they are absent in mice lacking glutamate release from thalamocortical terminals. Consequently, synapses remain immature. Expression of synapse-regulating genes and synaptic development is also altered when astrocyte signaling is blunted by diminishing calcium release from astrocyte stores. Single-nucleus RNA sequencing identified groups of astrocytic genes regulated by neuronal and astrocyte activity, and a cassette of genes that show layer-specific enrichment. Thus, the development of cortical circuits requires coordinated signaling between astrocytes and neurons, highlighting astrocytes as a target to manipulate in neurodevelopmental disorders.

Single-cell and single-nucleus RNA-seq uncovers shared and distinct axes of variation in dorsal LGN neurons in mice, non-human primates, and humans

Abundant evidence supports the presence of at least three distinct types of thalamocortical (TC) neurons in the primate dorsal lateral geniculate nucleus (dLGN) of the thalamus, the brain region that conveys visual information from the retina to the primary visual cortex (V1). Different types of TC neurons in mice, humans, and macaques have distinct morphologies, distinct connectivity patterns, and convey different aspects of visual information to the cortex. To investigate the molecular underpinnings of these cell types, and how these relate to differences in dLGN between human, macaque, and mice, we profiled gene expression in single nuclei and cells using RNA-sequencing. These efforts identified four distinct types of TC neurons in the primate dLGN: magnocellular (M) neurons, parvocellular (P) neurons, and two types of koniocellular (K) neurons. Despite extensively documented morphological and physiological differences between M and P neurons, we identified few genes with significant differential expression between transcriptomic cell types corresponding to these two neuronal populations. Likewise, the dominant feature of TC neurons of the adult mouse dLGN is high transcriptomic similarity, with an axis of heterogeneity that aligns with core vs. shell portions of mouse dLGN. Together, these data show that transcriptomic differences between principal cell types in the mature mammalian dLGN are subtle relative to the observed differences in morphology and cortical projection targets. Finally, alignment of transcriptome profiles across species highlights expanded diversity of GABAergic neurons in primate versus mouse dLGN and homologous types of TC neurons in primates that are distinct from TC neurons in mouse.

Microglia, Cytokines, and Neural Activity: Unexpected Interactions in Brain Development and Function

Intercellular signaling molecules such as cytokines and their receptors enable immune cells to communicate with one another and their surrounding microenvironments. Emerging evidence suggests that the same signaling pathways that regulate inflammatory responses to injury and disease outside of the brain also play powerful roles in brain development, plasticity, and function. These observations raise the question of how the same signaling molecules can play such distinct roles in peripheral tissues compared to the central nervous system, a system previously thought to be largely protected from inflammatory signaling. Here, we review evidence that the specialized roles of immune signaling molecules such as cytokines in the brain are to a large extent shaped by neural activity, a key feature of the brain that reflects active communication between neurons at synapses. We discuss the known mechanisms through which microglia, the resident immune cells of the brain, respond to increases and decreases in activity by engaging classical inflammatory signaling cascades to assemble, remodel, and eliminate synapses across the lifespan. We integrate evidence from (1) in vivo imaging studies of microglia-neuron interactions, (2) developmental studies across multiple neural circuits, and (3) molecular studies of activity-dependent gene expression in microglia and neurons to highlight the specific roles of activity in defining immune pathway function in the brain. Given that the repurposing of signaling pathways across different tissues may be an important evolutionary strategy to overcome the limited size of the genome, understanding how cytokine function is established and maintained in the brain could lead to key insights into neurological health and disease.

Optic Nerve Regeneration: How Will We Get There?

BACKGROUND

Restoration of vision in patients blinded by advanced optic neuropathies requires technologies that can either 1) salvage damaged and prevent further degeneration of retinal ganglion cells (RGCs), or 2) replace lost RGCs.

EVIDENCE ACQUISITION

Review of scientific literature.

RESULTS

In this article, we discuss the different barriers to cell-replacement based strategies for optic nerve regeneration and provide an update regarding what progress that has been made to overcome them. We also provide an update on current stem cell-based therapies for optic nerve regeneration.

CONCLUSIONS

As neuro-regenerative and cell-transplantation based strategies for optic nerve regeneration continue to be refined, researchers and clinicians will need to work together to determine who will be a good candidate for such therapies.